The invention relates to a device for detecting the tire pressure of a motor vehicle tire.
For safety reasons, the tire pressure of motor vehicle tires must be checked regularly. For this purpose, devices for detecting the tire pressure are arranged in every tire. These devices comprise at least one pressure sensor that measures the tire pressure. The measured value is then transmitted in a signal via an antenna with the aid of a transmitter to a vehicle-side receiver. Transmission can occur intermittently at predefined time intervals or after receipt of a prompting signal sent by a vehicle-side transmitter. The energy required to send the pressure signal back to the vehicle can be transmitted to the device along with the prompting signal.
The pressure signal can also include an individual tire identification. This identification is stored in the electronic unit of each tire. The pressure signals received by the vehicle-side receivers are supplied to a central vehicle-side evaluation unit in which each signal is evaluated and the measured pressure is compared with permissible reference values. If the measured pressure differs from the reference value, a display is activated, which prompts the driver to take suitable measures (changing the tire or adding air). As a safety measure, several measurements may be taken to determine a mean value or a time-dependent value, which is then compared with the reference value, which can also change as a function of time.
A prior art device (EP 0 657 836 B1) comprises an electronic unit, which is arranged on a tire of the vehicle. A coil that is mounted along the circumference of the tire and is completely fixed to the inner tire wall or the inner tire tread is connected to this electronic unit. The electronic unit is used to measure the pressure inside the tire. Upon receipt of a prompting signal emitted by the vehicle-side antenna, the pressure signal is radio transmitted via the antenna to the vehicle-side receiving antenna.
When the vehicle is traveling (i.e. the wheels are rotating), the tires are deformed on the one hand due to the load of the vehicle (roller leveling in the contact area) and on the other hand due to the centrifugal force. Since the antenna in the prior art device is fixed directly to the interior of the tire wall, the antenna is subject to a large mechanical load, particularly if the tire is exposed to additional substantial shocks, e.g. from driving over a curbstone or a speed ramp. The prior art antenna is firmly affixed to the elastic tire. As a consequence, there is a risk that the antenna will break, since it is less elastic than the rubber tire.
The object of the invention is to provide a device for detecting the tire pressure in a motor vehicle tire, the electronic components of which will reliably function over a relatively long service life.
According to the invention, this object is attainted by a device for detecting the tire pressure of a motor vehicle tire with the features of claim 1. This device comprises an electronic unit, which is connected to an antenna. Both the electronic unit and the antenna are arranged on a slightly elastic support in the form of an annular disk, the inner circumference of which is mounted to the tire while the outer circumference side freely projects into the interior of the tire.
Due to this single-sided fixation to the tire and the elastic flexible support, vibrations and mechanical loads that act on the tire are transmitted only in an attenuated manner to the support and its electronic unit as well as the antenna. Consequently, the mechanical loads applied to these electronic components during operation of the vehicle are minor.
In the device according to the invention, the antenna is arranged around the wheel rim and approximately coaxially to the wheel axle. This axially symmetrical arrangement largely avoids additional imbalances. Since the antenna is arranged over the circumference of the tire, data can be readily transmitted back and forth between the antenna and a vehicle-side antenna if the vehicle-side antenna is arranged in proximity to the wheel.
Through this support, the entire weight of the support plus the electronic unit and the antenna is distributed about equally over the circumference, so that only minor imbalances associated with the support act on the wheel and thus the driving stability.
The mounting of the support to the tire can be integrated in the tire production process. The support and thus the electronic unit are then permanently associated with the tire. Such a support can also be added or replaced for each tire whenever the tire is changed or repaired.
Advantageous embodiments of the invention are set forth in the dependent claims. For instance, the antenna can be arranged as a coil with one or several turns along the circumference of the support and can be largely surrounded by the support material. As a result, the antenna itself is protected from environmental influences. If the support is made of a flexible material, vibrations that are due to mechanical effects on the tire and are transmitted to the support are dampened. As the wheels rotate, there is no compression of the ring. Even if the tire is exposed to extreme loads (e.g. in cornering) or extreme compression (moving the tire over a sharp edge) and the tire contacts the support as a result, the support due to its flexibility or elasticity, can absorb the mechanical load to a large extent such that the antenna and the electronic unit are largely protected.
The support with its antenna is preferably mounted to the inner tire wall facing the vehicle side in the area of the tire bead. Thus, the antenna is arranged as close as possible to a vehicle-side antenna, which is typically arranged in proximity of the wheel or the suspension strut. As a result, little energy is required to transmit the signals. In devices with their own power supply (battery), the power supply is consequently conserved and lasts longer. In devices without a battery (transponder method), less energy needs to be transmitted to the device to enable the return of the pressure signal.
Since the two antennas are arranged close together, the data can be transmitted after modulation with low carrier frequencies. These signals have a short range in any case. This has the advantage, however, that the devices in the other tires of the vehicle do not pick up the pressure signals. As a result, it is possible uniquely to assign each device in a corresponding tire to the respective wheel position.
The support can be produced by injection molding and the antenna and the electronic unit can be molded into the support during this process, so that they are surrounded and protected by the support material. To enable the tire pressure to be measured exactly by a pressure sensor of the electronic unit, the support material is provided with a hole in the area of the electronic unit, which forms an air duct between the air in the tire and the pressure sensor.
The support can also be made of a flexible, electrically insulating material that encloses the antenna and the electronic unit after suitable cutting and folding of the material. Here, too, care must be taken that an air duct is provided to the electronic unit, so that the pressure in the tire can be measured accurately and without distortion.
The support preferably has an approximately rectangular profile cross-section. Depending on the number of turns of the coil antenna and the size of the electronic unit and its enclosure, the profile cross-section of the support can also be T-shaped or approximately L-shaped.
To avoid excessive mechanical loading of the support during travel, i.e. wheel rotation, the support is provided with a approximately ring-shaped reinforcing element in the area of the tire bead. As the wheel rotates, this reinforcing element mechanically stabilizes and relieves the support by reducing the tensile load on the support.